The present disclosure relates to a laser diode device.
A high power and ultrashort pulse laser diode device that is composed of GaN compound semiconductor and that has light emitting wavelength of 405 nm band has been expected to be a light source for a volumetric optical disc system expected as a next generation optical disc system displacing the Blu-ray optical disc system or has been expected to be a light source needed in the medical field, the bio imaging field and the like. As a method of generating short pulse light in the laser diode device, mainly two methods that are gain switching method and mode-locking method are known. The mode locking is further categorized into active mode locking and passive mode locking. To generate light pulses based on the active mode locking, an external resonator is configured by using a mirror or a lens, and further radio frequency (RF) modulation should be added to the laser diode device. Meanwhile, in the passive mode locking, light pulses are able to be generated by simple direct current drive by using self pulsation operation of the laser diode device.
To enable the self pulsation operation of the laser diode device, the laser diode device should be provided with a light emitting region and a saturable absorption region. Based on arrangement state of the light emitting region and the saturable absorption region, the laser diode device is able to be categorized into SAL (saturable absorber layer) type and WI (weakly index guide) type in which the light emitting region and the saturable absorption region are arranged in the vertical direction, and bi-section type in which the light emitting region and the saturable absorption region are apposed in the resonator direction. The bi-section type laser diode device is known from, for example, Japanese Unexamined Patent Application Publication Nos. 2004-007002, 2004-188678, and 2008-047692. It is known that a bi-section type GaN laser diode device has larger effect of saturable absorption and is able to generate light pulse with a narrower width than in the SAL type laser diode device.
In general, the bi-section type GaN laser diode device includes a laminated structure in which a first compound semiconductor layer that has a first conductivity type and is composed of GaN compound semiconductor, a third compound semiconductor layer that has a light emitting region and a saturable absorption region composed of GaN compound semiconductor, and a second compound semiconductor layer that has a second conductivity type different from the first conductivity type and is composed of GaN compound semiconductor are sequentially layered, a strip-shaped second electrode formed on the second compound semiconductor layer, and a first electrode electrically connected to the first compound semiconductor layer. The second electrode is separated into a first section obtaining forward bias state by applying a direct current to the first electrode through the light emitting region and a second section adding electric field through the saturable absorption region from the first electrode by an isolation trench.